The present disclosure relates to image forming apparatuses including a photosensitive drum as an image carrier.
There is a type of image forming apparatus which includes a plurality of photosensitive drums associated with respective different color toners. This type of image forming apparatus has, for example, a structure employing the tandem method. In the image forming apparatus employing the tandem method, even when, for example, only a black toner is used to form an image on a paper sheet serving as a sheet material for image transfer, not only the photosensitive drum associated with the black toner but also the photosensitive drums associated with cyan, magenta, and yellow toners are rotated. In this case, the image forming apparatus does not charge the surfaces of the photosensitive drums associated with the cyan, magenta, and yellow toners (i.e., the photosensitive drums for color printing), so that the performance of these photosensitive drums will degrade. Then, the image forming apparatus has to repeat a few minutes of charging and a few minutes of static elimination in order to recover the charging performance of the photosensitive drums. This presents a problem of failure to form a color image on the next paper sheet just after the formation of a black-and-white image on the previous paper sheet. In addition, when charging the surfaces of the photosensitive drums, the image forming apparatus may cause electric breakdown (leakage) in the photosensitive drums due to inrush current at the instant of application of a charging bias.
As a solution to these problems, an image forming apparatus is proposed in which in forming a black-and-white image on a paper sheet using a black toner, the surfaces of the photosensitive drums for color printing are charged more weakly than during the formation of images on them and the static charge on the surfaces is then eliminated, thus enabling the formation of a color image on the next paper sheet just after the formation of a black-and-white image on the previous paper sheet.